The prevalence of osteoporosis is high in United State with about 10 million people over the age of 50 already having the disease and another 34 million at risk for developing osteoporosis. The release of first ever Surgeon General's report on the topic of bone health emphasizes the enormity of this public health problem. Development of low-cost and effective strategies is important for prevention of osteoporosis and to reduce osteoporotic fractures. A simple inexpensive strategy to prevent osteoporosis is adequate nutrition with calcium and vitamin D. Serum 25OHD is now accepted as the objective measure of vitamin D nutrition. There is a growing understanding that serum 25-hydroxy vitamin D (25OHD) concentrations of at least 30-32 ng/ml are needed for optimal bone health at which serum parathyroid hormone (PTH) concentrations reach a minimum. There are no systematic prospective dose response studies aimed at determining the optimum amount of vitamin D intake required to maintain optimum serum 25OHD levels in the population which will help in determining the estimated average requirement (EAR) and recommended dietary requirement (RDA) for vitamin D. The proposed proposal is aimed at filling this gap by concentrating on the high risk group of postmenopausal women between age 60-85 years. We hypothesize that increasing serum 25OHD to a level greater than 30 ng/ml or more in 97% of the study subjects with vitamin D supplements in winter will reduce serum PTH in the high risk group of vitamin D insufficient (serum 25OHD <20 ng/ml) postmenopausal women in the presence of an adequate intake of calcium. We postulate that the dose of vitamin D that will achieve a serum 25OHD of >30 ng/ml in 97% of the subjects is approximately 4400IU/day which is well above the suggested adequate intake of 400-600 ID recommended for vitamin D for the elderly. In a one year double blind, randomized prospective clinical trial, we will examine the dose response effect of supplementation with different doses of vitamin D3 in postmenopausal Caucasian and African American women with hypovitaminosis D in winter, on serum 25OHD and PTH which are the primary outcome measures. Also, the effect of various vitamin D3 supplement doses on calcium absorption, 1,25-dihydroxy vitamin D levels, serum calcium, bone markers, bone mineral density and falls will be studied and constitute our secondary outcome measures. We expect that the results from this proposal will add important information about the RDA for vitamin D for postmenopausal women who are more susceptible to osteoporosis. More work to determine the RDA for vitamin D has been recommended by the Panel on Calcium and Related Nutrients of the Food and Nutrition Board. The results from this study will also help in designing future larger clinical trials of the effect of vitamin D on falls and fractures.